1. Field of the Invention
The present invention relates to a cylindrical sleeve comprising an SiC fiber-reinforced SiC composite material, which is capable of exhibiting excellent durability even under a severe condition such that it is irradiated with radioactive rays or radiation, etc. The composite material sleeve according to the present invention can suitably be used, particularly, as a component (or element) to be disposed in a nuclear reactor, a component constituting a control rod for controlling a nuclear reactor, a high-heat flux component for constituting a nuclear fusion reactor, a component for constituting a high-temperature heat exchanger, etc.
2. Related Background Art
A ceramic material has been well known as a material which is excellent in heat resistance. However, it is a relatively recent event that such a ceramic material has further overcome its disadvantage of "brittleness (or fragility)" so that it may retain a high strength even under a high-temperature condition. In addition, because of the "hard and brittle" characteristics of the ceramic material, it has required a special measure or device to use a ceramic material having a desired heat resistance and high strength as a structural material, i.e., to precisely process or form the ceramic material into a desired shape.
For example, with respect to a "sleeve" which is a hollow cylindrical member, when a monolithic ceramic material is used, the ceramic material is liable to cause brittle fracture during the processing, forming or machining thereof. Accordingly, it is difficult to prepare a long sleeve (e.g., one having a length of about 1 m) with a wall thickness of 0.2 to 0.4 mm.
When a carbon fiber-reinforced carbon composite material is used as the material for constituting a sleeve, it is possible to prepare the above-mentioned long sleeve. However, the carbon fiber-reinforced carbon composite material has a problem such that it is liable to cause a carburizing reaction with a cladding (or covering tube) comprising stainless steel (i.e., diffusion and permeation of carbon into the surface layer of the stainless steel).
When a heat-resistant steel is used as the material constituting a sleeve, it is also possible to prepare a long sleeve. However, the heat-resistant steel has a problem such that it is liable to cause a carburizing reaction with a boron carbide (B.sub.4 C) pellet as a neutron absorber (or neutron absorbing material).
When an alumina fiber-reinforced CMC (ceramic matrix composite material) is used as a sleeve material, such a CMC material has a problem such that it is liable to cause volume expansion or swelling when irradiated with neutrons.
Heretofore, as a control rod for controlling a nuclear reactor such as fast breeder reactor, one comprising a stainless steel cladding and a B.sub.4 C pellet which has been inserted into the cladding.
When the control rod disposed in a nuclear reactor is irradiated with neutrons, the temperature of the control rod becomes higher, and the stainless steel constituting the cladding can be carburized by B.sub.4 C so that it becomes brittle, whereby the cladding can be broken. In addition, as the B.sub.4 C absorbs neutrons, it generates helium and is expanded, and the B.sub.4 C pellet is broken due to a thermal stress caused by the heat generation and the resultant fragments are moved so that they fill the gap between the B.sub.4 C and the stainless steel cladding. When the B.sub.4 C pellet is continuously irradiated with neutrons in such a state, the resultant stress may act between the thus expanded B.sub.4 C and the stainless steel tube, whereby the stainless steel cladding can be damaged in some cases.
In consideration of these circumstances, for the purpose of preventing the fragments of boron carbide B.sub.4 C from moving in the cladding, there has been proposed a structure wherein a thin-wall pipe (usually, referred to as a "shroud") comprising austenite stainless steel, ferrite steel-stainless steel, etc., is disposed in the above-mentioned stainless steel cladding so that the shroud covers the entire length of the neutron absorber pellet (see, Japanese Patent Publication (KOKOKU) No. Hei 6-31769 (i.e., 31769/1994)).
However, when the above-mentioned control rod equipped with the shroud is used for a long period of time, the shroud comprising the stainless steel has a problem such that it is reacted with the boron carbide pellet and loses its ductility due to the carburization thereof, etc., thereby to lower its function or performance as a shroud tube.